Studies continue to determine cellular and molecular mechanisms underlying the interactions of natural and synthetic estrogenic compounds with various target tissues (developing genital tract, gonads, liver, and mammary gland) resulting in permanent alterations in differentiation including dysmorphology and neoplasia. Diethylstilbestrol (DES) is used as a model environmental estrogen. In vivo studies in which DES exposure occurs during development (prenatal and/or neonatal) of the genital tract have shown various benign/malignant lesions in male/female mice similar to those reported in humans prenatally exposed to DES. Immunocytochemical and molecular genetic characteristics of DES-induced murine tumors are being compared with those seen in human malignancies. Descendants of tumor- bearing mice have been followed for reproductive assessment and potential increased susceptibility to tumors. Results show an increased incidence of reproductive tract tumors in the female and male descendants of mice developmentally exposed to DES. The ontogeny of lactoferrin (LF), complement C3, and estrogen receptor(ER) alpha and beta in the developing reproductive tract have been mapped in vivo/in vitro to determine early estrogenic responses and the role of ER alpha and beta in normal and altered development induced by estrogens. To determine if the ER is playing a role in the induction and/or progression of uterine neoplasia, transgenic mice that overexpress ER alpha and mice expressing an altered form of ER alpha have been treated with DES. Neonatal DES treatment causes accelerated onset and increased severity of uterine neoplasia in mice that over express ER alpha. Dysmorphology in the genital tract of mice exposed to DES includes persistent gene expression of uterine LF and various growth factors including TGF-alpha and EGF which may be preneoplastic markers. Other markers of long-term abnormalities are being sought as well as possible effects of DES that might occur at low doses of exposure. Altered methylation patterns have been identified in the estrogen responsive lactoterrin gene following developmental DES exposure. Similar changes in other genes are being investigated. Since questions remain concerning the roles of growth factors, growth inhibitors, and receptors in the induction of long term permanent abnormalities, studies continue to identify additional human/mouse genes that characterize the different physiological states of normal or neoplastic tissue. Studies determining cell proliferation and apoptotic rates in target tissues exposed to DES have given further insights into mechanisms of DES-toxic effects. Tamoxifen and genistein effects on developing estrogen target tissues are being studied and their dysmorphology/neoplastic potential compared to DES. Other reported endocrine disrupting chemicals are being tested for estrogenic potency and compared in multiple bioassays. Sensitive markers of estrogenicity are being sought in both mice and rats. Together these studies suggest a complex pattern of altered differentiation and development in hormone-responsive target tissues and provide important clues for mechanisms involved in the development of neoplasia.